1. Field of the Invention
The present invention relates to a digital television broadcast signal receiver for receiving a digital television signal such as an Advanced Television Systems Committee (ATSC) signal.
2. Description of the Related Art
In the North America where ATSC digital television broadcast (hereinafter referred to as TV broadcast) is provided, television signals are transmitted from variously located broadcast stations at big cities or their suburbs in the plains. From the viewpoint of a user who receives such TV broadcast, television signals come from various directions. Accordingly, it is necessary for a user to adjust the orientation of an antenna so as to aim the antenna at a broadcast station transmitting a desired TV program. For this purpose, multi-directional antennas such as smart antennas are put into practical use.
The Electronic Industries Association (EIA)-909 standard provides that a smart antenna has sixteen receiving directions corresponding to the respective directions by which a circle is divided into sixteen. Various types of multi-directional antennas or smart antennas are commercially available. For example, a multi-directional antenna is known that is configured to rotate with a motor so as to change its active receiving direction. Another multi-directional antenna is known that includes a plurality of antenna units pointing in different directions and electronic switches, and makes one of the antenna directions active by turning on and off the electronic switches (without rotation of the antenna itself).
Some of television broadcast signal receivers to be connected to such a multi-directional antenna as described above are designed to be capable of automatically adjusting the orientation of a multi-directional antenna. On the other hand, in the case where such an antenna is placed in a room for example, the receiving condition tends to frequently change. Therefore, it is also needed to allow a user to know the current receiving condition.
There are known methods of displaying the receiving condition of an antenna by on-screen display function. For example, Japanese laid-open patent publication 2002-320165 discloses a method of receiving signals on a channel by one of a plurality of tuners while receiving signals on other channels by another one of the tuners to determine and store receiving conditions for the respective channels and displaying the stored receiving conditions in list form as the need arises. Japanese laid-open patent publication 2003-101897 discloses a method of displaying together the received signal strengths of two or more kinds of signals transmitted in different ways without the need to switch into a mode for receiving each kind of signals. Japanese laid-open patent publication 2003-234663 discloses a method of measuring the levels of signals received from two satellites and displaying the measured signal levels in the form of a horizontal bar graph on the screen.
As described above, the ATSC digital broadcasting in the U.S.A. may cause television signals to come from every direction. In order to receive such television signals, a multi-directional antenna such as a smart antenna is needed as well as a digital television broadcast signal receiver (hereinafter, abbreviated as a digital TV broadcast signal receiver) conforming to the EIA-909 standard that is configured to be capable of switching an active receiving direction of the multi-directional antenna to any one of the sixteen directions.
Since the position of an antenna at a broadcast station or transmission tower does not change generally, a digital TV broadcast signal receiver can scan the sixteen directions for each channel to set one of the directions in which TV signals on each channel can be received in the best condition as a best receiving direction for the channel. Some of digital TV broadcast signal receivers have an automatic scanning function for selecting a receiving direction of an antenna in which TV signals on a user-selected channel can be best received.
A conventional digital TV broadcast signal receiver is configured to, in initial setting of a multi-directional antenna or smart antenna, take time to scan all the directions for each channel to determine a best direction for reception of signals on each channel and a maximum value of received signal strengths on each channel. The conventional digital TV broadcast signal receiver then stores the best receiving direction and the maximum received signal strength in a memory provided therein so that, when a channel is selected by a user, the antenna can be promptly oriented to receive signals on the selected channel based on the stored data. However, when displayed video images are somewhat degraded during signal reception, a user may desire to manually change the orientation of the antenna to obtain better quality images. In such a case, the conventional digital TV broadcast signal receiver does not allow a user to easily change the orientation of the antenna but requires another automatic scanning, which is time consuming. In addition, there are cases where image quality is degraded while the orientation of the antenna should be appropriate under normal circumstances without any interference. For example, image quality may be degraded due to interference signals or ghosts during signal reception while the received signal level is not low. Further, in the case where an indoor antenna is used, image quality may be degraded every time a car passes outdoors. In such a case, even when the antenna is reoriented through another scanning, the receiving condition often remains not improved while much time is spent in the scanning.
In the above case where signal reception is in poor condition or interference signals or ghosts are present, there is a possibility that image quality can be improved or the interference signals or ghosts can be avoided or reduced by orienting the antenna in another direction in which the received signal level is equivalent to that in the original position, if any.
In the above described cases, it is convenient if a user is allowed to readily change the orientation of the antenna. In order to manually reorient the antenna in such a case, a technique is needed that informs a user of precise position of the antenna while the antenna is rotated, i.e., allows a user to reorient the antenna while checking how much the antenna is rotated with consideration given to a case where the antenna is to be returned to the original position.